Multiple-input, multiple-output (MIMO) wireless communication systems use multiple antennas at both the transmitter and the receiver to improve performance. Each antenna at the transmitting station provides an individual path to each antenna at the receiving station.
Transmit beamforming (TxBF) can improve the range of a MIMO system and can also improve the data transmission rate. In a process referred to as beamforming, a TxBF-enabled device can utilize the knowledge of a MIMO channel to properly weight and combine the raw signals such that the reception in the receiving station is improved. By using beamforming, the received signals are, in effect, stronger and can therefore support higher data rates over longer distances.
A TxBF-enabled transmitting station (also referred to as a beamformer) relies on knowledge of the channel between it and a receiving station (also referred to as a beamformee) to help focus the transmissions to a specific location. A sounding signal from the beamformee can help the beamformer estimate channel characteristics such as the gain and phase response for each transmit and receive antenna pair between the beamformer and beamformee. If the channel is reciprocal, then the beamformer can use the training symbols it receives from the beamformee to make a channel estimate suitable for computing the transmit steering matrix.
However, differences in the gain and phase characteristics of the beamformer's transmit and receive chains can affect channel reciprocity. Hence, a calibration procedure is employed to calculate a correction matrix that can be applied at the beamformer to compensate for the differences between the transmit and receive chains. If both the beamformer and beamformee are calibrated, then reciprocity is restored in the baseband-to-baseband response in both the forward and reverse channels between the two stations.
The IEEE 802.11n wireless networking standard specifies that calibration of MIMO systems should be performed, but it does not specify how to perform the calibration. The calibration process can increase loads on the processing, power and memory resources of a TxBF-enabled transmitter. Improving the efficiency of the calibration process can help reduce those loads.